Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes than both incandescent bulbs and fluorescent tubes. In addition, the efficiency of conversion of electricity to light has now reached the same levels as obtained in fluorescent light fixtures.
LED sources that are to replace conventional lighting generate significant amounts of heat that must be transferred to the environment without raising the operating temperature of the LEDs to levels that unacceptably decrease the efficiency or lifetime of the LEDs. The efficiency of conversion of electricity to light in an LED decreases with increasing temperature. Similarly, the lifetime of the LED also decreases with temperature. To replace conventional lighting, LED sources that dissipate 10 to 100 watts of heat are required. For example, an LED-based replacement for a fluorescent tube assembly having two 4-foot T12 tubes must dissipate more than 50 watts. To move this amount of heat from the LEDs without increasing the LED temperature to an unacceptable level presents design challenges and is the subject of numerous patents.
Typically, the LEDs are mounted on some form of heat-conducting structure that moves the heat from the LED dies to a second heat-dissipating structure that transfers the heat to the ambient environment, usually to the air that is in contact with a heat-dissipating surface. To maintain the temperature of the LEDs at acceptable levels, the heat-conducting structure is usually a layer of a metal such as copper or aluminum and held at an acceptable temperature by bonding the heat-conducting structure to a much larger surface that is used to dissipate the heat. In some applications a fan is utilized to move the air across the larger area heat-dissipating surface to reduce the area needed to dissipate the heat.
In situations in which an existing luminaire is being upgraded by replacing a conventional light source such as a number of fluorescent tubes with an LED-based light source, the problems associated with mounting the replacement light source that includes the heat-conducting structure to the final heat-dissipating structure can present significant challenges. The final heat-dissipating structure can be part of the existing luminaire or a separate structure that is part of the replacement light source. If the replacement light source matches the form factor of the conventional light source that is being replaced, the area that is available for the heat-dissipating structure is often too small to provide adequate heat transfer without incurring substantial costs to provide a large area heat-transfer surface that fits in a restricted space that was previously occupied by the conventional light source. If the heat-dissipating structure is to be part of the existing luminaire, attachment of the heat-dissipating structure to the existing luminaire can present significant challenges.
For example, consider an existing fluorescent light fixture that has a number of T8 fluorescent tubes that are to replaced by LEDs. If the replacement light source is to look like a conventional T8 fluorescent tube, the heat-dissipating structure must fit into a cylindrical tube that has a diameter of 1 inch and a length equal to that of the fluorescent tube being replaced. This constraint limits the power output of the replacement light source and/or increases the cost by requiring a more complex heat-dissipating structure that includes fins or the like that must fit into one half of the cylindrical replacement source.
If the replacement light source does not match the form factor of the light source being replaced, a new method for attaching the replacement to the existing fixture is needed. Methods that utilize adhesives or mechanical fasteners present a new set of problems. Mechanical fasteners often require that new holes must be drilled in the existing fixture to insert one part of the fastener so that a screw can be used to attach the replacement light source. In many cases, the existing light source cannot be conveniently removed to make these alterations, as it is attached to structural members of the building. Hence, the alterations to the existing fixture must be made in place which incurs significant costs. In addition, the type of fasteners that can be utilized are limited by the fact that all sides of the existing fixture cannot be conveniently accessed.
If adhesives are used, replacing the new light source when it fails can present additional problems. Furthermore, the surfaces of many existing fixtures are painted or otherwise coated with compounds that are not necessarily good surfaces for adhesive attachment. In addition, the adhesive must withstand the temperature cycling of the new light source.